The present invention relates to optical power control in optical systems, and more particularly to a variable optical attenuator (xe2x80x9cVOAxe2x80x9d) employing polarization maintaining fiber.
Fiber optic systems often require precise control of optical signal levels entering various system components. For example, a laser may produce an optical xe2x80x9ccarrierxe2x80x9d signal to be modulated by an electrical signal in a modulator, fiber-coupled to the laser. It is often desirable to control the output power of the laser, because of its inherently varying level. The variable, fiber optic attenuators of the type disclosed in the above-incorporated U.S. Patent Applications, e.g., the applications entitled xe2x80x9cBLOCKLESS FIBER OPTIC ATTENUATORS AND ATTENUATION SYSTEMS EMPLOYING DISPERSION TAILORED POLYMERSxe2x80x9d and xe2x80x9cCONTROLLABLE FIBER OPTIC ATTENUATORS EMPLOYING TAPERED AND/OR ETCHED FIBER SECTIONSxe2x80x9d offer certain advantages such as direct fiber coupling, low insertion loss, spectral uniformity, and automated feedback control. The principle underlying these attenuators is access to the evanescent field of the optical signal transmitted in a fiber, through a side surface of the fiber, by e.g., side polishing, tapering and/or etching an interior portion of the fiber. The laser-modulator combination discussed above will benefit from the power control capabilities of such attenuators.
Maintaining the polarization of the optical signal is often required, and a laser-modulator combination will often employ a certain xe2x80x9cspeciesxe2x80x9d of fiber, i.e., polarization maintaining (PM) fiber for this purpose. As discussed in section 6.4 of the 1998 Prentice-Hall book by Dennis Derickson entitled xe2x80x9cFiber Optic Test and Measurement,xe2x80x9d the term polarization maintaining refers to a class of highly linearly birefringent single mode fiber. PM fiber is typically used to guide linearly polarized light from point to point, for example, between a DFB (distributed feedback) laser diode and a lithium-niobate modulator in a high-speed telecommunication system, and in other specialized systems. Any attenuators employed in-line in PM systems must therefore also maintain polarization of the optical signal.
Fiber birefringence may be stress-induced, by placing the core between or within glass elements of different physical composition, or may originate with a purposeful asymmetry in the core geometry (form birefringence). Because the attenuators discussed above use portions of fiber which have their cross-sections intentionally modified, the polarization maintaining nature of the fiber used may be affected.
What is required, therefore, is a technique for preserving the polarization maintaining property of a fiber optic in an attenuator which, through intentional modification of the fiber structure, may otherwise affect this property.
The shortcomings of the prior approaches are overcome, and additional advantages are provided, by the present invention, which in one aspect relates to an attenuator for attenuating optical energy, having a portion of a polarization maintaining fiber optic through which the optical energy is transmitted, the portion of the fiber having a side surface through which at least some of said optical energy can be controllably extracted. The portion of the fiber is modified to allow some of said optical energy to be extracted while maintaining the polarization of the transmitted optical energy. A controllable material is formed over the side surface of the fiber optic for controllably extracting said optical energy. The controllable material controllably extracts the optical energy according to a changeable stimulus applied thereto. A housing is provided enclosing the portion of the fiber optic and the controllable material, and the portion of the fiber optic is suspended within the housing to be substantially thermally insulated by surrounding air or other effective thermal insulator in the housing.
The portion of the polarization maintaining fiber optic is symmetrically modified by removal of material therefrom in a direction perpendicular to either a fast or slow axis of the fiber. The portion of the polarization maintaining fiber optic may be polished to effect said removal. The polarization maintaining fiber optic may be a panda-type fiber with two cylindrical rods, wherein at least a portion of either or both cylindrical rods is removed in said direction perpendicular to either a fast or slow axis of the fiber.
The attenuator may further comprise a controllable heating/cooling source in operative contact with the controllable material to change the temperature thereof and a thermal sensor for sensing the temperature of the controllable material. The controllable material may have its dispersion properties tailored in accordance with those of the fiber across a given wavelength band.
Various system implementations are also disclosed, including a closed loop feedback system and/or a laser for generating the optical energy along with a modulator for modulating the optical energy.
The present invention also extends to methods for forming and using the attenuator and its associated systems.